1. Field of the Invention
The present invention relates to optical waveguides.
2. Description of Related Art
Many modern devices incorporate optical waveguides. Generally, an optical waveguide is any structure having the ability to guide the flow of radiant energy, such as light, along a path parallel to the structure's optical axis and having the ability to contain the energy within or adjacent to the structure's surface. Examples of optical waveguides include optical fibers, light pipes, and the like. Such optical waveguides often comprise glass, acrylic, or the like.
FIG. 1A depicts an exemplary, conventional, frustoconical, optical waveguide 101. Such frustoconical optical waveguides are used, for example, to collect light from a larger aperture and concentrate the light at a smaller aperture. Light enters optical waveguide 101 at an optical entrance 103. If a light wave entering optical waveguide 101 at optical entrance 103 is substantially collimated, i.e., every ray of the light wave is substantially parallel to one another, and the collimated light wave is parallel to an optical axis 105 of optical waveguide 101, substantially all of the collimated light wave exits optical waveguide 101 at an optical exit 107. In other words, the amplitude of the light wave entering optical waveguide 101 at optical entrance 103 is substantially the same as the amplitude of the light wave exiting optical waveguide 101 at optical exit 107.
For example, as shown in FIG. 1A, a light ray 109 of the collimated light wave enters optical waveguide 101 at optical entrance 103 and propagates substantially unattenuated through optical waveguide 101 and optical exit 107 without encountering an outer surface 111 of optical waveguide 101. A light ray 113 of the collimated light wave enters optical waveguide 101 at optical entrance and propagates through optical waveguide 101 but encounters outer surface 111, generally at 115. At 115, light ray 113 is totally, internally reflected and propagates through optical exit 107.
Total internal reflection occurs when light is refracted or bent at a medium boundary enough to send it backwards, effectively reflecting the entire ray. When a light ray propagates across a boundary surface (e.g., outer surface 111) between materials with different refractive indices, the light ray will be partially refracted at the boundary surface and partially reflected. However, if the angle of incidence (e.g., angle a) is shallower (closer to the boundary) than the critical angle, then the light ray will stop crossing the boundary altogether and, instead, totally reflect back internally within optical waveguide 101. The critical angle is the angle of incidence wherein a light ray is refracted so that the light ray travels along the boundary between the media and is defined as:
            θ      c        =                  sin                  -          1                    ⁡              [                              n            1                                n            2                          ]              ,wherein θc is the critical angle, n1 is the refractive index of the less dense material, and n2 is the refractive index of the more dense material. Total internal reflection can only occur where a light ray propagates from a denser medium to a less dense medium, i.e., from the medium with a higher refractive index to a medium with a lower refractive index. For example, total internal reflection will occur when a light ray propagates from glass to air, but not when the light ray propagates from air to glass.
In the example illustrated in FIG. 1A, as in all optical waveguides, optical waveguide 101 comprises a material having a higher refractive index than a medium 117 in which optical waveguide 101 is disposed. Since the angle of incidence a between light ray 113 and outer surface 111 is less than the critical angle for the interface or boundary between optical waveguide 101 and medium 117, light ray 113 is totally internally reflected within optical waveguide 101, as depicted in FIG. 1A, if outer surface 111 contains no optical defects. In a practical sense, however, outer surface 111 will contain optical defects and, thus, the amplitude of light ray 119 is somewhat attenuated at each encounter with outer surface 111. Accordingly, a light ray 118 entering optical waveguide 101 will exit optical waveguide 101, but only after encountering, and being reflected by, outer surface 111 a plurality of times. Because of the inherent optical defects in outer surface 111, the amplitude of light ray 118 is more attenuated in optical waveguide 101 than the amplitude of light ray 113 or light ray 109. Correspondingly, the amplitude of light ray 113 is more attenuated in optical waveguide 101 than light ray 109.
Even if the light wave entering optical waveguide 101 through optical entrance 103 is not collimated light, the amplitude of the light wave exiting optical waveguide 101 through optical exit 107 may be substantially undiminished. For example, as shown in FIG. 1B, a light ray 119 enters optical waveguide 101 at an angle b and is refracted at an angle c at optical entrance 103 because medium 117 exhibits a lower refractive index than optical waveguide 101. Note that, optical axis 105 is perpendicular to optical entrance 103. Snell's law characterizes optical refraction, in that:n1 sin(θ1)=n2 sin(θ2),wherein:
n1 is the refractive index of a first material (e.g., medium 117);
θ1 is the angle of incidence (e.g., angle b) of a light ray in the first material (e.g., ray 119 in medium 117);
n2 is the refractive index of a second material (e.g., light ray 119 in waveguide 101); and
θ2 is the angle of refraction (e.g., angle c) of the light ray in the second material.
Light ray 119 encounters outer surface 111 of optical waveguide at angle d, generally at 121. Because, in the illustrated example, angle d is less than the critical angle for the boundary between optical waveguide 101 and medium 117, light ray 119 is totally internally reflected within optical waveguide 101 and exits optical waveguide 101 through optical exit 107 substantially unattenuated, except for attenuation due to optical defects in outer surface 111.
At greater angles of incidence at optical entrance 103, however, the amplitude of a light ray may be further attenuated as the light ray propagates through optical waveguide 101. For example, as illustrated in FIG. 1C, a light ray 123 strikes optical entrance 103 at an angle of incidence e and is refracted at optical entrance 103 at an angle of refraction f. Light ray 123 propagates further through optical waveguide 101, encountering outer surface 111 generally at 125 and at 127. At 127, however, light ray 123 strikes outer surface 111 an angle of incidence g, which exceeds the critical angle for the interface or boundary between optical waveguide 101 and medium 117. Accordingly, light ray 123 is split, with a first portion of light ray 123 being refracted into medium 117 as light ray 123′ and a second portion of light ray 123 being reflected in optical waveguide 101 as light ray 123″.
At each successive encounter of the remaining, reflected portion of light ray 123 (e.g., light ray 123″) with outer surface 111 of optical waveguide 101, the remaining, reflected portion of light ray 123 is further split into a refracted component, propagating into medium 117, and a reflected component, reflected into optical waveguide 101. The remaining, reflected portion or component of light ray 123 is further split because the angle of incidence of the remaining, reflected portion of light ray 123 with outer surface 111 is greater than the critical angle of the interface or boundary between optical interface 101 and medium 117. Thus, light rays having larger angles of incidence upon optical entrance 103, such as light ray 123, are not totally, internally reflected within optical waveguide 101 but are substantially attenuated, if not completely attenuated (as illustrated in FIG. 1C), as light ray 123 propagates through optical waveguide 101.
As mentioned above, conventional, frustoconical, optical waveguides are often used to gather light at a larger aperture (e.g., optical entrance 103) and focus or concentrate the gathered light at a smaller aperture (e.g., optical exit 107). However, such optical waveguides are ineffective in applications wherein the light rays entering the larger aperture that are to be focused or concentrated at the smaller aperture are not totally, internally reflected within the optical waveguide. In some implementations, conventional, frustoconical, optical waveguides are used to concentrate or focus light onto a detector or other such electronic sensor. However, in some operational situations, the amplitude of the light exiting the optical waveguide onto the detector may be insufficient for the detector to properly operate, because a preponderance of the light rays striking the optical entrance of the optical waveguide have large angles of incidence at the optical entrance, as discussed above. In such situations, the orientation of the optical waveguide must be changed with respect to the direction at which the light rays are propagating, which may cause, for example, packaging problems. It should be noted that conventional, frustopyramidal, optical waveguides suffer from the same problems noted above with respect to conventional, frustoconical, optical waveguides.
While conventional, right-cylindrical, optical waveguides, such as a right-cylindrical, optical waveguide 201 of FIG. 2, do not generally suffer the problems described above, particular implementations of such optical waveguides do present other problems. For example, as illustrated in FIG. 2, optical waveguide 201 is disposed in housing 203 such that light, represented by an arrow 205, propagates through opening 207 defined by housing 203. Packaging constraints, however, may require optical waveguide 201 to be positioned such that an optical entrance 209 of optical waveguide 201 is not flush with housing 203. In implementations wherein housing 203 experiences high velocity fluid flow adjacent thereto, as represented by an arrow 211, a gap 213 between housing 203 and optical waveguide 201 may induce severe turbulence and undesirable turbulence-induced forces on housing 203. One example of such an implementation is in airborne or waterborne vehicles, such as missiles, rockets, aircraft, drones, torpedoes, and the like.
While there are many designs of optical waveguides well known in the art, considerable shortcomings remain.